[Ada] Improve generated code for initialization of atomics

[Ada] Improve generated code for initialization of atomics

This patch makes the temp for initialization of an atomic variable be
constant.

No change in behavior; no test.

2019-08-20  Bob Duff  <duff@adacore.com>

gcc/ada/

	* freeze.adb (Is_Atomic_VFA_Aggregate): Make the temp for
	initialization of the atomic variable be constant. This is
	cleaner, and might improve efficiency.

From-SVN: r274728